How does water serve as suitable microbial habitat?
pH – Aquatic microorganisms usually grow at a pH ranging from 6.5-8.5. The pH of seawater is 7.5-8.5. Marine microbes grow best on culture media whose pH has been adjusted to 7.2-7.6. The pH of lakes and rivers generally shows a wide range. Example, several archeabacteria have been isolated from salt lakes in Africa where the pH is 11.5. There are some archeabacteria which are found to grow at a pH of 1.0 or less.
Nutrients – The quantity and kind of organic and inorganic materials present as nutrients in a water body determines the microbial growth. Nitrates and phosphates promote the growth of algae. However, significantly high concentration of these nutrients can lead to overgrowth of algae to such an extent which may deplete the oxygen supply of the water body, suffocating all other forms of life. The quantity of nutrients in a water body is called nutrient load. The nutrient load at near-shore water, which receives domestic wastewater, is high in amounts of organic and inorganic wastes and also variable. On the contrary, open-sea system has low and stable nutrient load comparatively. Industrial wastes also contribute some antimicrobial substances such as mercury (Hg) and other heavy metals. They inhibit the growth of some microbes while simultaneously promoting the growth of others. Example – Pseudomonads can survive in an environment containing Hg. They convert Hg into methylmercury which is a volatile compound that escapes into the atmosphere removing Hg from the aquatic environments.
How do microorganisms adapt to aquatic environments?
Very small size – Most oceanic microbes are less than 0.2µm in diameter. They are called ultramicrobacteria. Development of very fine filtration and direct counting methods like epifluorescent microscopy led to the discovery of these microbes. Cells with a larger surface are than their total intracellular volume can maximize their nutrient uptake and can thus grow more quickly than other larger microbes. Thus, majority of the microorganisms growing in oligotrophic open ocean are between 0.3-0.6 µm. They have evolved to increase the surface area with respect to volume in order to survive in those conditions.
On the other hand, a marine microbe, Thiomargarita namibiensis (which means ‘sulfur pearl of Namibia’) is considered the world’s largest bacterium till date (100-300 µm diameter). They use sulphide and nitrate as the electron donor and acceptor, respectively. It stores nitrate in intracellular vacuoles which may occupy 98% of the organism’s volume. Elemental sulphur appear near the edge of the cell in a thin layer of cytoplasm. Nitrate penetrates the anoxic, sulphide-containing muds only during storms. Thus, in between storms, the organism utilizes the stored nitrate as an electron acceptor.
Ability to link and use resources – Another important adaptation of microbes is that they can link and use resources in separate locations or that are available in the same location only for a short interval. One widely studied microbe is Thioploca, which lives in bundles enclosed by a common sheath. They are found in nutrient upwelling areas along the coast of Chile, where oxygen-poor but nitrate-rich waters are in contact with sulphide-rich mud. They can glide 5-15 cm deep into the sediments.
Attachment – Several microbes adhere to surfaces. Examples – Sphaerotilus, Leucothrix, Caulobacter and Hyphomicrobium. There exist a number of gliding bacteria such as Flexithrix and Flexibacter, which move over surfaces onto which organic matter is adsorbed. These bacteria allow formation of complex biofilm.
Microscopic fungi also survive in aquatic habitats such as chytrids. They adapt to life in water by producing motile asexual spores with a single whiplash flagellum. They decompose dead and decaying organic matter and attack algae.
Ingoldian fungi produce unique tetraradiate forms. The tetraradiate conidium forms on a vegetative mycelium that grows inside decomposing leaves. When the hyphae differentiate into aerial mycelia, conidia are released into water which are then transported and frequently present in surface foam. When the come in contact with new leaves, they attach and start growing on them. This adaptation allows the fungi to process organic matter. Many aquatic insects feed only on leaves containing fungi.
